1. Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process with hydrocarbon hydroisomerization in the synthesis reactor. More particularly the invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process, wherein the synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor by circulating it down through a downcomer reactor in the synthesis slurry, in which the liquid reacts with hydrogen in the presence of a monolithic hydroisomerization catalyst.
2. Background of the Invention
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas, which comprises a mixture of H2 and CO, up into a hot reactive slurry in a hydrocarbon synthesis reactor, in which the slurry comprises synthesized hydrocarbons which are liquid at the synthesis reaction conditions and in which is dispersed a particulate Fischer-Tropsch type of catalyst. The H2 and CO react in the presence of the catalyst and form hydrocarbons. The hydrocarbon liquid is continuously or intermittently withdrawn from the synthesis reactor and pipelined to one or more downstream upgrading operations. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. The downstream upgrading includes fractionation and conversion operations, typically comprising hydroisomerization, in which a portion of the molecular structure of at least some the hydrocarbon molecules is changed. It would be an improvement if the synthesized hydrocarbon slurry liquid could be at least partially hydroisomerized to reduce its pour and melt points within the synthesis reactor and without the need for a separate hydroisomerization facility, to make it more transportable by pipeline, before it is transferred to downstream operations.
The invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process in which the synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor by circulating it down through one or more downcomer reactors in the synthesis slurry, in which the liquid reacts with hydrogen in the presence of a hydroisomerization catalyst and preferably a monolithic hydroisomerization catalyst, to hydroisomerize the liquid which is then passed back into the slurry body in the synthesis reactor. The slurry liquid, which comprises synthesized hydrocarbons that are liquid at the synthesis reaction conditions, comprises mostly normal paraffins and the hydroisomerization reduces its pour and melt points, thereby making it more pumpable and pipelinable. By downcomer reactor is meant a downcomer containing the hydroisomerization catalyst in its interior and that circulation of slurry from the slurry body down through the downcomer reactor is produced all or mostly by density-difference hydraulics, in which the density of the slurry flowing down through the downcomer reactor is greater than the surrounding slurry body in the synthesis reactor. Slurry densification is achieved by removing at least a portion of the gas bubbles from the slurry, thereby densifying it before it passes down the downcomer reactor. The one or more downcomer reactors may each comprise a simple, substantially vertical, hollow fluid conduit or pipe open at its top and bottom and are immersed in the slurry body in the synthesis reactor. Except for the absence of a hydroisomerization catalyst and means for injecting a hydrogen treat gas into its interior, the simple type of downcomer having a slurry gas bubble removing means at it top disclosed in U.S. Pat. No. 5,382,748 the disclosure of which is incorporated herein by reference, is an example of means which can be modified to be useful as a downcomer reactor in the process of the invention.
The process comprises contacting hot slurry from the slurry body in the synthesis reactor with means for removing gas bubbles, and preferably gas bubbles and at least a portion of the particulate solids from the slurry liquid which densifies it, with the densified slurry and a hydrogen treat gas passed into the interior of the one or more downcomer reactors and then back into the surrounding slurry body. The hydroisomerization catalyst is located in the interior of the downcomer reactor and comprises the hydroisomerization reaction zone, in which the hydrogen reacts with the slurry hydrocarbon liquid to hydroisomerize at least a portion of it and produce a hydroisomerized liquid of reduced pour point. The hydroisomerized hydrocarbon liquid of reduced pour point then passes out of the downcomer and back into the surrounding slurry body in the synthesis reactor. This enables hydroisomerizing the slurry liquid (i) inside the synthesis reactor and (ii) while the synthesis reactor is producing hydrocarbons, but without interfering with the hydrocarbon synthesis reaction. The concentration of hydroisomerized hydrocarbon liquid in the synthesis reactor continues to increase until equilibrium conditions are reached. When the reactor reaches equilibrium it is possible for the slurry liquid being removed from it to comprise mostly hydroisomerized hydrocarbons of reduced pour point. In some cases, no further hydroisomerization of the liquid hydrocarbon product withdrawn from the synthesis reactor is necessary. Thus, the process of the invention will reduce and in some cases even eliminate the need for a separate, stand-alone hydroisomerization reactor and associated equipment downstream of the synthesis reactor. If a downstream hydroisomerization reactor is needed, it will be smaller than it would be if the synthesized hydrocarbon liquid passed into it was not at least partially hydroisomerized. While all of the hydroisomerized hydrocarbon liquid is typically returned back into the surrounding slurry body in the synthesis reactor with which it mixes, in some embodiments a portion of the hydroisomerized liquid may be passed from the downcomer reactor, out of the syntheses reactor to downstream operations.
The gas bubble and preferably the slurry gas bubble and particulate solids removal means is also located in the slurry body in the synthesis reactor and may comprise the same or separate means. While various filtration means may be used to separate the slurry liquid from at least a portion of the catalyst and any other particles, before the slurry is passed down into the hydroisomerization zone, in the practice of the invention the use of filtration means may be avoided by using known slurry solids reducing means that do not is employ filtration. Gas bubble and solids removal means suitable for use with the present invention are known and disclosed in, for example, U.S. Pat. Nos. 5,866,621 and 5,962,537, the disclosures of which are incorporated herein by reference. In addition to the ""748 patent referred to above, simple gas bubble removing means are also disclosed in U.S. Pat. Nos. 5,811,468 and 5,817,702, the disclosures of which are also incorporated herein by reference. While gas bubble and solids removal means may or may not be part of the downcomer reactor, in these four patents the gas bubble and the gas bubble and solids removal means are immersed in the slurry body and comprise the slurry entrance at the top of the downcomer. As mentioned above, removing gas bubbles from the slurry densifies it and, if properly employed in connection with feeding it down into and through the downcomer reactor (e.g., the slurry is densified sufficiently above the external hydroisomerization zone), provides a density-difference driven hydraulic head to circulate the slurry from the slurry body in the synthesis reactor, down into and through the internal downcomer reactor and back into the surrounding slurry body. Removing gas bubbles from the slurry prior to hydroisomerization also reduces the CO and water vapor content of the flowing fluid, which could otherwise react with the hydroisomerization hydrogen and also adversely effect the hydroisomerization catalyst. A monolithic hydroisomerization catalyst having substantially vertical fluid flow channels and a minimal solid cross-sectional area perpendicular to the flow direction of the fluid minimizes the pressure drop of the fluid flowing down and across the catalyst surface. Removing catalyst and other solid particles, such as inert heat transfer particles, from the slurry upstream of the hydroisomerization zone, reduces scouring of the monolithic catalyst and plugging of the hydroisomerization reaction zone.
The invention comprises a slurry Fischer-Tropsch hydrocarbon syntheses process in which synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor during hydrocarbon synthesis, by circulating slurry from the slurry body in the synthesis reactor down through a hydroisomerization zone in a downcomer reactor immersed in the slurry body, in which the slurry hydrocarbon liquid reacts with hydrogen in the presence of a hydroisomerization catalyst. Slurry circulation between the downcomer reactor and slurry body is achieved by the densification resulting from the gas bubble removal. At least a portion of the slurry liquid is hydroisomerized and this reduces its pour point. The hydroisomerized slurry leaves the downcomer reactor and all or most of it passes back into the surrounding slurry body with which it mixes. Preferably the hydroisomerization catalyst comprises a monolithic catalyst and at least a portion of both solids and gas bubbles are removed from the slurry before it contacts the hydroisomerization catalyst. More specifically the invention comprises a hydrocarbon synthesis process which includes hydroisomerizing hydrocarbon liquid produced by the synthesis reaction while the hydrocarbon liquid is being produced from a synthesis gas, the process comprising the steps of:
(a) passing a synthesis gas comprising a mixture of H2 and CO into a slurry body comprising a three-phase slurry in a slurry Fischer-Tropsch hydrocarbon synthesis reactor, in which the slurry comprises gas bubbles and a particulate hydrocarbon synthesis catalyst in a slurry hydrocarbon liquid;
(b) reacting the H2 and CO in the presence of the catalyst at reaction conditions effective to form hydrocarbons, a portion of which are liquid at the reaction conditions and comprise the slurry hydrocarbon liquid;
(c) contacting a portion of the slurry from the slurry body with means for removing gas bubbles, to form a densified slurry reduced in gas bubbles whose density is greater than that of the slurry comprising the slurry body in the synthesis reactor;
(d) passing a hydrogen treat gas and the densified slurry into a hydroisomerizing zone in one or more downcomer reactors immersed in the slurry body in the synthesis reactor, in which the hydrogen and hydrocarbon slurry liquid react in the presence of a preferably monolithic hydroisomerization catalyst to form a hydrocarbon liquid of reduced pour point, and
(e) passing all or a portion of the pour point reduced liquid back into the surrounding slurry body.
While the liquid is being synthesized and hydroisomerized in the synthesis reactor, a portion is continuously or intermittently withdrawn and sent to downstream operations. It is preferred that at least a portion, and more preferably as much as possible of the particulate solids are removed from the slurry, before it is passed down into the hydroisomerizing zone.